Proving a Helix using Frenet Formulas

Given a wire with constant curvature (k) and constant torsion (t), prove how these define a helix using Frenet Formulas below:

dB/ds = -t(s)N
dT/ds = k(s)N
dN/ds = tB - kT

I know that I need to prove that dN/ds x N is a constant vector to conclude that N(s) is planar to start with, but every way I try to work it, I keep getting dN/ds x N = -tT - kB and I have no idea how to go from there.